Surface acoustic wave devices, known as SAW devices, have many uses in the UHF and VHF frequency ranges. SAW devices have been especially useful as impedance elements, resonators, and band-pass filters in these frequency ranges. Typical SAW devices have a substrate with at least a surface layer of piezoelectric material and surface acoustic wave transducers in interdigitated form disposed on the piezoelectric surface. The transducers convert an electrical signal to surface acoustic waves propagating on the piezoelectric surface. Such transducers may also include grating structures.
Yet a further concept for implementing a unidirectional single-phase transducer is set forth in U.S. Pat. No. 4,162,465. The approach described therein employs electrodes of varying widths to achieve the desired unidirectional characteristics. This approach has the advantage of being single level. However, in general, the gaps and electrode widths in such a structure are required to be significantly smaller than in conventional SAW transducers. This is a significant drawback to its practical implementation.
SAW devices are compact, lightweight, robust, and, because they are a planar technology, are economical to manufacture. They can be mass-produced using the same techniques developed so successfully for the production of silicon integrated circuits. A wide variety of analog signal processing functions can be achieved with SAW devices. Among other applications, they are currently used in pulse compression radar systems as receiver band-pass filters or as resonators for stabilizing oscillators in numerous applications. They have replaced many of the coils, capacitors, and metal cavities of conventional radio frequency systems, removing the need for hand alignment and dramatically improving the reliability and performance of such systems. They have simultaneously resulted in significant reductions in both size and cost.
However, several problems are associated with the prior art surface acoustic wave transducers. One of the problems occurs because the transducer electrodes cause internal reflections which distort the transducer output and the shape of the input conductance which, in most cases, is undesirable. Another problem occurs when the transducer is used in filter applications. Triple transit distortion is caused by regeneration reflections between the transducers.
In order to eliminate triple transit distortion, three-phase, group type, and single-phase devices are used to cause a greater amount of radiation in one direction in the crystal than in the reverse direction and thus form unidirectional transducers. One such device is disclosed in commonly assigned U.S. Pat. No. 4,902,925, commonly known as the "Hopscotch", and is incorporated herein by reference in its entirety. This structure employed a group type sampling with all electrode widths being .lambda./4. The first level of the transducer, by virtue of the electrode groupings, has no net internal reflections. Unidirectionality is achieved only by the addition of a second level metalization or by the inclusion of grooves with this structure.
Another concept for a single-phase unidirectional transducer is set forth in commonly assigned U.S. Pat. No. 4,910,839 in which, with unique crystal orientations, a simple two-electrode-per-wavelength transducer exhibited unidirectional characteristics. With this device the sense of directionality is determined by the material properties of the crystal substrate and overlay material, rather than by the transducer configuration as with other approaches.
Also, in commonly assigned U.S. Pat. No. 5,073,763, incorporated herein by reference in its entirety, a class of group-type single-phase unidirectional transducers were disclosed that can obtain unidirectional characteristics on conventional or natural crystal orientations. The sense of unidirectionality can be reversed by a change to the second level metalization or, again, by the inclusion of grooves. This transducer operated with 3/8.lambda. and 5/8.lambda. sampling. In these devices, the single-level versions are reflectionless. Thus, unidirectional characteristics are obtained only from the two-level structure.
Associated with many of these transducers are reflector gratings that have the same reflection problems of the transducers. It would be advantageous to have a grating structure for a surface acoustic wave device that has unidirectional characteristics with only single-level metal and practical electrode widths and gap dimensions.